Build a pulsometer/oximeter for birds

Need a wearable for small animals 1.5 kgs (like cats, chickens and small dogs) to measure pulse (200 to 500 bpm) and oximetry (70% to 90%) 4 times a day. The ideal is to send the data once captured to a server, but it can be stored. like an apple watch... (without screen) Max dimensions 20x20x5 mm and 25 g (bat included). It should last 3 weeks. Casing will be required after testing the pulse/oximetry functions.